In recent years, new materials have emerged as potential candidates for magnetoresistive (MR) devices. In the case of "giant" magnetoresistance (GMR), interfacial spins modulate the electron transport both in artificially engineered nanostructural materials consisting of magnetic multilayers and in properly crafted granular alloys. The "colossal" magnetoresistance (CMR) analogs are manganite perovskites with a magnetic-field-driven metal-insulator transition at tens of kOe. These compounds have the general formula A.sub.1-x B.sub.x MnO.sub.3+.delta., where A=La, Nd and B=Bu, Sr, Ca, Pb. These compounds undergo a transition into a ferromagnetic conducting state for a narrow composition range around x=0.33 where the resistivity is .about.10.sup.4 .mu..OMEGA.-cm (comparable in magnitude with narrow-gap semiconductors). Near room temperature, the normalized MR can be very large for H of many tens of kOe, the scale of the magnetic-field-driven metal-insulator transition, but .DELTA..rho./.rho. falls precipitously at low fields.
The silver chalcogenides, Ag.sub.2 S, Ag.sub.2 Se and Ag.sub.2 Te, are classic superionic conductors of technological import as solid electrolytes. In their high temperature (.alpha.) phase, electrical transport proceeds via a well-orchestrated coupling of microscopic lattice distortions to the ion migration dynamics. Below T.about.400 K, there is a structural transformation into the .beta. phase, where the cation sublattice freezes into a semiconducting state with characteristic energy gaps as small as a few hundredths of an eV. Magnetic fields are commonly applied during the growth process to aid anion vaporization, but have a negligible effect on the electronic response in the solid. In fact, a noise-limited magnetoresistance (MR) in intrinsic .beta.-Ag.sub.2 Se has been reported.